Flowmeters provide information about materials being transferred through a conduit. For example, a flowmeter may provide a direct indication of the velocity and volume of the material being transferred through the conduit. Mass flowmeters measure the mass of the material, and density flowmeters measure the density of the material.
One type of flowmeter is a vortex-shedding meter, or vortex flowmeter. Vortex flowmeters operate using techniques based on a vortex shedding phenomenon known as the Karman effect. In the vortex flowmeter, a bluff body is placed in the conduit (e.g., a pipe) in the path of the material passing through the conduit. The bluff body sheds Karman vortices in its wake. These vortices cause fluctuating pressure variations that then are amplified and processed. The frequency of the Karman vortices is directly proportional to the flow rate of the material, as are the pressure variations occurring during vortex generation.
To produce a flow measurement, the flowmeter first determines the vortex shedding frequency, ωv, in rad/sec. The flowmeter also converts the reading into a flow, f, in units of 1/sec, through use of f=Kmωv, where Km is the appropriate meter factor. This meter factor Km (or K-factor) relates the frequency of generated vortices to the flow rate.
Conventional vortex flowmeters can accurately measure the vortex shedding frequency down to a threshold low flow rate of material passing through the conduit. Below this threshold, accuracy of conventional measurement becomes unreliable. This primarily results because the amplitude of the vortex shedding pressure is much smaller at low flow rates, such that the signal is much noisier due to harmonic components caused by pressure pulsations from, for example, the impeller blades of the pump(s). Therefore, at low flow rates, the decreased amplitude of the vortex shedding pressure and the increased effect of noise on the measurement system lead to greater uncertainties in the flow rate measurement and/or slower determinations of flow rate.